Liquid level detection

ABSTRACT

A method and apparatus for detecting the presence of a liquid level at a monitor point along the depth of a borehole utilizes a diaphragm pressure switch for sensing the pressure of a captive air column in a measurement tube extending from the pressure switch to a predetermined point below the monitor point. In use, a rising liquid level in the borehole seals off the open lower end of the measurement tube from the earth atmosphere and allows the rising liquid level to compress the captive air column. In a preferred embodiment a visual signal is activated by the pressure switch when the rising liquid level in the borehole reaches the monitor point.

This is a continuation of application Ser. No. 419,348, filed Oct. 10,1989, now U.S. Pat. No. 4,972,705.

This invention relates generally to sensing the presence of a liquidlevel. In one aspect it relates to a liquid level detection apparatusand method for use in an injection well having a borehole. In anotheraspect it relates to an apparatus and a method for determining whetheror not a liquid level is present at a preselected depth in a borehole.

BACKGROUND OF THE INVENTION

In recent years the domestic oil industry has been placing greater andgreater emphasis on the secondary (and tertiary) recovery of oil. Thisis so because (1) exploration costs have increased, (2) large domesticreserves are rarely found, and (3) crude oil imports are required tomeet demands. These reasons coupled with ever more stringentenvironmental protection requirements have resulted in a continualdomestic crude oil shortage.

Waterflood, the oldest assisted oil recovering method, remains the mostcommonly used method in the United States for increasing domesticsupplies. In waterflood, water is introduced into an oil reservoirthrough an injection well. As used herein an injection well is a wellutilized in secondary recovery of petroleum, in which a fluid such asgas or water is injected into an oil reservoir to provide supplementalenergy to drive the oil remaining in the reservoir to the vicinity ofproduction wells.

In the practical application of injection wells, a new well may bedrilled or a producing well may be converted to an injection well. Aninjection well therefore includes the injection tubing string inaddition to the packer, casing, and possibly a production casing. Inoperation of an injection well, it is generally impossible to make astatement as to the optimum injection rate for flooding a reservoirbecause of the wide range of rock and fluid characteristics in oilreservoirs. It is recognized, however, that excessive injectionpressure, which can lead to uncontrolled reservoir fracturing, should beavoided. On the other hand the injection rate and the accompanyinginjection pressure must be sufficient to maintain production.

A problem that arises with injection wells is that sometimes the waterlevel rises into the borehole and fills the space between the varioustubes and the casing. In active wells a leak in the injection tubing orpacking can fill the annulus between the injection tubing and thecasing, and injection water will eventually seep into the formationsurrouding any leak in the casing. Another possible scenario is that aleak in the injection tubing or packing, which allows injected water toback up into the annulus, would be accelerated by an excessive injectionrate. Water levels can also rise in inactive wells due to leaks in thecasings.

In the event of a leak in the casing, injection water can seep into thesurrounding formation and contaminate underground sources of drinkingwater which are at depths below the leak in the casing. Consequently theUnited States Environmental Protection Agency has enforced mechanicalintegrity regulations for water injection wells. These regulationsrequire non-leaking casings so as to safeguard the underground sourcesof drinking water.

Since casing leaks are expensive to repair, and because injection wellshaving casing leaks are especially numerous in older oil fields whereextended periods of waterflood have been employed, a method fordetermining the presence of water at a level in the borehole of aninjection well which would not threaten underground sources of drinkingwater, and which would allow time for steps to be taken to preventfurther rise of water in the borehole, would be highly desirable. Thedetection method would thereby safeguard underground sources of drinkingwater without making expensive repairs necessary.

Accordingly, it is an object of this invention to provide apparatus andmethod for detecting the presence of a liquid level in a borehole whichis reliable, inexpensive, and relatively simple to implement.

It is a further object of this invention to detect the presence ofliquid in a borehole without employing electrical devices in theborehole.

It is still another object of this invention to protect undergroundsources of drinking water from contamination by injection water used insecondary oil recovery.

SUMMARY OF THE INVENTION

In accordance with the present invention a first predetermined pointalong the depth of a borehole is monitored for the presence of a liquidlevel. The method comprises the steps of:

a) installing a measuring tube in the borehole, wherein the lower end ofthe measuring tube is positioned in the borehole at a secondpredetermined point along the depth of the borehole, and wherein thesecond predetermined point is below the predetermined first point;

b) sealing off the upper end of the measuring tube from the earth'satmosphere while the measuring tube is in communication with the earth'satmosphere through the lower end.

c) allowing a quantity of air in the measuring tube to be trapped whenthe liquid level in the borehole contacts the open lower end of themeasuring tube so as to seal off the lower end of the measuring tubefrom the earth's atmosphere; and

d) establishing a signal, responsive to the actual pressure of thetrapped quantity of air, the signal indicating the presence of theliquid level at the first predetermined point.

In another aspect, apparatus for practicing this invention comprise

a) a measuring tube having a predetermined length;

b) means for installing said measuring tube in a borehole, wherein thelower end of said measuring tube is positional at a second predeterminedpoint along the depth of said borehole; and

c) means for establishing a signal responsive to said liquid level insaid borehole, wherein said signal is activated when said water level insaid borehole rises above said first predetermined point.

In a preferred embodiment of this invention a point in the borehole of awater injection well is monitored. The selected point is below thelowest level of any underground sources of drinking water in thesurrounding area. Monitoring the selected point is therefore aneffective method of safeguarding the underground sources of drinkingwater, since steps such as starting or shutting down a pump, or closinga valve can be taken before the injection water in the borehole rises toa level which actually threatens the underground sources of drinkingwater. Further in the preferred embodiment of the present invention avisual signal is displayed to alert operators of the injection wellsbefore the liquid level reaches a threatening level.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a water injection well withassociated apparatus for utilizing the method of the present invention.

FIG. 2 is a schematic illustration of a pneumatically actuatedindicating device according to the invention.

FIG. 3 is a schematic illustration of an electrically actuatedindicating device according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

While the invention is described in terms of sensing a water level in aninjection well utilized for secondary oil recovery it is not limited tosensing water. Any fluid level which rises in a borehole and which has adensity sufficient to compress a column of air can utilize the method ofthe present invention.

Referring to FIG. 1 there is illustrated a schematic view of aninjection well which is equipped for sensing the presence of a liquidlevel at a preselected point along the depth of the borehole. The groundlevel 10 is indicated with a bore 12 extending downwardly therefromuntil the producing formation 14 is encountered. As is typical, the boreis cased with steel casing 16 for maintaining the integrity of the bore.Hung from the surface within the steel casing 16 is a production tubestring 18 and an injection tube string 20 with the packing 21 producinga seal between the tubular strings 18 and 20. In normal operation of aninjection well when a fluid such as water is injected into anunderground oil reservoir 14, valve 22 is at least partially open andpump 24 flows water from water supply 26 via conduit 28 and valve 22into the injection tube string 20. The water supply 26 may be anysuitable supply. Generally speaking the water supply for a water floodshould be extensive, inexpensive, free from bacteria, suspended solidsand oxygen and unreactive to any clays in the reservoir itself.

In any waterflood of an oil reservoir the possibility exists for a waterlevel to rise into the bore 12 through leaks in the injection tubing 20or packing 21, and the rising water level in the borehole 12 will beaccelerated if excess injection water is pumped. Further illustrated inFIG. 1, by the arrow 30, is the lowest level of any underground sourceof drinking water which must be protected.

In accordance with the invention as illustrated in FIG. 1 a measuringtube 32 is installed in the bore 12 by any suitable means such asrigidly hanging from the surface. An open end 34 of the measuring tube32 is positioned at a point below the drinking water level 30 as will bemore fully explained hereinafter. The measurement inlet port of apressure switch 38 is attached to the upper end 36 of the measuring tube32 while the lower end 34 is in open communication with the earth'satmospheric pressure. The pressure switch 38 seals off the upper end 36of the measuring tube 32 from externally applied pressures. Themeasuring tube 32 may be of any convenient diameter such as, forexample, a 3/8 inch tube. Generally copper or stainless steel tubes canbe suitably employed for the measuring tube 32.

Assuming that the water level 42 in the bore 12 is initially well belowthe open end 34 the measuring tube 32, but that the water level 42,rising in the bore 12, traps a quantity of air in the tube 32. Thenfurther elevation of fluid level 42 will compress the trapped aircaptured in measuring tube 32 in proportion to the elevation of thefluid level 42 above the lower end 34 of the tube 32. When the risingpressure in tube 32 reaches the predetermined pressure level that willactivate the pressure switch 38, a suitable signal, preferably a visualsignal, is activated. The visual signal may be an electric light;however, in a preferred embodiment the visual indicator is a flag 56that is elevated above surrounding structure by, for example, activatingan air piston 48, as shown in FIG. 2. The pneumatically actuated flagsignal is particularly attractive for unattended wells in that the flagmay easily be observed in daylight while driving by an oil field in acar and further in that no external energy is used until the signal isactivated.

The pressure switch 38 may be any suitable switch. The essentialrequirements being that the switch 38 operate at a low pressure levelsuch as, for example, 2 or 3 psig and that the diaphragm volume of theswitch 38 be negligible compared to the volume of the measuring tube 32.In the preferred embodiment the switch 38 is one that depends on thedeflection of a diaphragm, referenced to atmospheric pressure, to changeits length and transmit this motion to an output shaft such that apredetermined or set point pressure corresponds to a predeterminedmovement of the output shaft. Switch 38 may also be a switch used toenergize and deenergize electrical circuits as a function of the processpressure and a predetermined set point. Operation of the differentswitches is illustrated in FIG. 2 and FIG. 3.

Referring now to FIG. 2, where like reference numerals refer to the samepart in each of the drawing figures in which the part appears, there isschematically illustrated a deflection type pressure switch 38 inaccordance with the invention. The pressure switch 38 illustrated inFIG. 2 transmits motion to an output shaft 42 which in turn activates alever action micro valve 44 by moving the lever 43. The micro valve 44connects an air supply 46 to an air operated cylinder 48 via conduits 50and 52. The air pressure supplied to the cylinder 48 pushes the rod 54upwardly thereby displaying a signal flag 56 which is attached to rod54. Air operated cylinder 48 is supported near the upper end of supportpipe 58 by a fitting sandwiched between pipe ends at coupler 55. Thevertically positioned pipe 58 positions the cylinder 48 in an elevatedposition so that the flag signal is visible from a distance when aircylinder 48 operates and pushes the flag 56 out of the end of the pipe58.

Referring now to FIG. 3 there is illustrated a pressure switch 38a ofthe type for activating or deactivating an electrical device and wherethe pressure switch 38a operates with reference to a predetermined setpoint. In operation, switch 38a changes from an "ON" position to an"OFF" position depending on the actual value of the process pressure. Asillustrated in FIG. 3, a voltage source such as a battery 60 and anindicating lamp 62 are connected in series with the pressure switchterminal 64. With this arrangement the lamp is illuminated when theprocess pressure reaches the monitored level. The invention, however,contemplates operation of other electrical devices connected to thepressure switch terminal 64, for example, an electric motor for the pump24 could be turned "ON" or "OFF" by the pressure switch 38.

To further illustrate the invention, Table 1, set forth below, is acalculated example of the depth of a monitored point in a well bore andthe corresponding base of the deepest underground source of drinkingwater to be protected. Also shown in Table 1 is the length of ameasuring tube required to operate pressure switches having a set pointof 2 psig and 3 psig. This Table 1, which is based on calculations usingthe ideal gas law, is constructed to satisfy the particular requirementsof a regulating agency. In Table 1 the depth of the open lower end 34 ofthe measuring tube 32 is about 14% below the monitor point when using a2 psi pressure switch and is about 21% below the monitor point whenusing a 3 psi pressure switch.

                  TABLE 1                                                         ______________________________________                                        ILLUSTRATIVE DEPTH REQUIREMENTS                                               All depths are given in feet.                                                 BASE OF                                                                       PROTECTED MONITOR    MEAS. TUBE  MEAS. TUBE                                   SOURCE    POINT      3 psi Switch                                                                              2 psi Switch                                 ______________________________________                                        100       200        242         228                                          200       300        362         342                                          300       400        483         455                                          400       500        604         569                                          500       600        725         683                                          600       700        846         797                                          700       800        967         911                                          800       900        1088        1025                                         ______________________________________                                    

In summary the method of the present invention provides means forinsuring that a visual or other suitable warning signal is displayed ifthe water level in the borehole of an injector well approaches a levelthat could threaten an underground source of drinking water, so as toallow time for remedial action before the underground source of drinkingwater is actually threatened.

The invention has been described in terms of a presently preferredembodiment as illustrated in FIGS. 1-3. Specific components which can beused in the practice of the invention as illustrated in FIG. 1, such aspressure switch 38, a lever action micro valve 44 and air cylinder 48,having a sufficient stroke length, are each well known, commerciallyavailable components such as are described at length in Perry's ChemicalEngineers Handbook, 5th Addition Chapter 22 McGraw-Hill, and arecommercially available from suppliers such as McMaster-Carr, Chicago,Ill.

While the invention has been described in terms of the presentlypreferred embodiment, reasonable modifications and variations arepossible by those skilled in the art and such modifications andvariations are within the scope of the described invention and theappended claims.

That which is claimed is:
 1. Apparatus for detecting the presence of aliquid level at a first point along the depth of a borehole, whereinsaid first point is selected for monitoring as a safe level forunderground water in said borehole, said apparatus comprising:(a) ameasuring tube having a length for extending from a reference surface toa second point, wherein said second point is a predetermined distancebelow said first point in said borehole; (b) means for installing saidmeasuring tube in said borehole, wherein the lower end of said measuringtube is positionable in said borehole at said second point; and (c)means for establishing a visual signal responsive to said liquid levelin said borehole, wherein said visual signal is significantly elevatedabove the earth's surface.
 2. Apparatus in accordance with claim 1,wherein the lower end of said measuring tube is positioned underatmospheric pressure at said second predetermined point in saidborehole, and thereafter the upper end of said measuring tube is sealedoff, and wherein a rising liquid level seals off the lower end of saidtube, and thereby traps a quantity of air in said tube, and further therising liquid sufficiently compresses said quantity of trapped air so asto exceed a predetermined pressure set point, said means forestablishing said signal comprises:a pressure switch connected to theupper end of said measuring tube; wherein said pressure switch isresponsive to the pressure in said tube, and further wherein saidpressure switch is adjusted to operate at said predetermined pressureset point; and means for providing a visual indication responsive tooperation of said pressure switch.
 3. Apparatus in accordance with claim2, wherein said predetermined set point is representative of thepressure of said trapped quantity of air which is compressed by saidliquid level rising to said first predetermined point.
 4. Apparatus inaccordance with claim 2 wherein said means for providing a visualindication responsive to operation of said pressure switch comprises:anair operated cylinder having a shaft arranged for movement in a verticaldirection; a fabric flag attached to said shaft; a compressed airsupply; and a micro valve for connecting said air supply to said aircylinder when said pressure switch operates, wherein said flag is raisedby air supplied to said cylinder to provide said visual indication. 5.Apparatus in accordance with claim 4 wherein said pressure switchoperates in a pressure range of from 2 psig to about 3 psig. 6.Apparatus in accordance with claim 4 wherein said second point is fromabout 14% to about 20% deeper than said first point.
 7. Apparatus inaccordance with claim 2 wherein said means for providing a visualindication responsive to operation of said pressure switch comprises:anelectric lamp for signaling presence of said liquid level at said firstpredetermined point; a voltage source connected to said lamp through anelectrical switch; and wherein said electrical switch is an integralpart of said pressure switch.
 8. Apparatus in accordance with claim 2,wherein said means for installing said measuring tube comprises:meansfor rigidly hanging said measuring tube from the earth's surface. 9.Apparatus for protecting underground sources of drinking water fromcontamination caused by liquid in a borehole, by detecting the presenceof a liquid level at a first point along the depth of said borehole,said first monitor point being representative of the lowest level ofdrinking water deposited in the drainage area surrounding said borehole,said apparatus comprising:(a) a measuring tube having a length suitablefor vertical installation in said borehole, said measuring tubeextending from a reference surface to a second point, wherein saidsecond point is a predetermined distance below said first point; (b) apressure switch connected to the upper end of said tube, wherein saidpressure switch is adjusted to operate at a predetermined pressure setpoint; (c) means for rigidly hanging said measuring tube from saidreference surface so that the lower end of said measuring tube ispositioned in said borehole at said second point, and wherein a risingliquid level seals off the lower end of said measuring tube from theearth's atmosphere and thereby traps a quantity of air in said measuringtube and on further elevation said rising liquid level can sufficientlycompress said quantity of trapped air so as to exceed said predeterminedpressure set point; (d) means for providing a visual signal responsiveto the actual pressure of said trapped quantity of air, said visualsignal indicating presence of said liquid level at or above said monitorpoint; and (e) wherein said visual signal is significantly elevatedabove the earth's surface.